A. Field of the Invention
The present invention relates to a thermostable photopolymeric film for liquid crystal (LC) alignment.
B. Description of the Prior Art
To obtain a monodomain planar orientation for a liquid crystal (LC) cell, several techniques were known. For example, in "Alignment of Nematic Liquid Crystals and Their Mixtures", J. Cognard, Gordon & Beach Science Publishers, 1982, it is described that to obtain a uniform liquid crystal orientation, microgrooves are produced on the surface of the substrate by mechanically treating (also known as "rubbing") the polymer surface of the substrate. However, this technique has a disadvantage in that microgrooves inherently have defects, and theses defects are known to cause random phase distortions and light scattering, degrading display characteristics. Further, static electricities generated during the rubbing of the polymer surface are known to cause defects in active matrix displays. Yet further, with this technique, it is practically impossible to locally orient selected regions of the surface, each region with a different orientation.
Another known technique is to fabricate an LC cell orientation film from a copolymer of polyamide (or polyimide) class. Such a copolymer has aliphatic and aromatic (heterocyclic) fragments in the main chain and fluoro-alkyl groups C.sub.n H.sub.m F.sub.(2n+1-m) in the side chains, e.g. 3-fluoromethyl group CF.sub.3, where n is a natural number and m a natural number of zero (0) or 2n or less. Such a copolymer is described, for example, in a European patent no. EP 0 217 641 A2, Int. Cl.sup.4 G 02 F 1/133; C 09 K 19/30, G 02 F 1/137 (1986), published 08.04.87, Bulletin 87/15. However, to make such-a type of a fluoro-alkyl-containing polyamide (or polyimide) orientation film, the substrate surface still has to be rubbed. Therefore, this technique inherently has the above mentioned disadvantages.
Yet another technique is known as described, for example, in M. Schadt et. al., Jpn. J. Appl. Phys., vol. 31, part 1, No 7, pp. 2155-2164 (1992). According to this technique, to fabricate a photopolymeric orientation film, prepolymers were produced by a reaction of a polyvinylalcohol with a substituted cinnamic acid. The methoxy-group was used as a substituent in the para-position of the benzene ring of cinnamic acid molecules. The prepolymers were photopolymerized by irradiating a linearly polarized ultraviolet (UV) light thereon. The photopolymerization of the prepolymer caused a directed cross-linking of polyvinyl-4-methoxy cinnamate (PVCN-M) linear chains to form a new net polymeric structure with a higher order due to a double bond opening reaction in vinyl fragments of cinnamoyl molecules, i.e., 2+2 cycloaddtion reaction.
Such a polymeric orientation film described in Schadt et al. possesses an optical anisotropy and is known to be capable of planarly orienting standard LC modecules in an preferred axial direction perpendicular to the UV light polarization vector. However, the main disadvantage of such a photopolymeric orientation film is low thermostability.
It has been experimentally discovered that with this type of polymeric orientation films, if the clearing point T of nematic LC (NLC) in the LC cell is greater than 50.degree. C., distortions in the planar orientation appear when the ambient temperature is approximately 50.degree. C. On the other hand, if the clearing point T of NLC is smaller than 50.degree. C., when the ambient temperature reaches an isotropic phase (i.e., for NLC 5CB, for example, it is about 40.degree. C.), the initial orientation does not resume and a great number of disclinations appear, thus irreversibly distorting liquid crystal orientation.
The operating temperature range of the LC cell trends to increase, the clearing point of the LC in the LC cell consequently increases, for example, up to the temperature of 80.degree. C..about.100.degree. C. The LC cell having a photopolymeric film, liquid crystals should be filled into the LC cell at temperatures near the clearing point of the LC, to avoid disclination lines in the LC cell. A conventional photopolymer film which uses PVCN-M as a prepolymer can not withstand such high process and operating temperatures.